Postreactor

Structure for gas-phase reactions. In general, efficient deflectors at the inlet and collectors at the outlet of the reactor are needed. A monolithic postreactor seems to be mature technology that requires only a dedicated catalyst. Such a relatively cheap reactor can be installed when retrofitting the plant or designed for new plants. 

As we pointed out in Chapter 1, the preceding describes the postreactor finishing operation. The pellets are then shipped to be compounded, namely, blended with fillers, colorants, or other polymers, where after melting, mixing, and reacting, they form physical... 

In the manufacturing process of most polymers, there is a need to separate and remove undesirable, residual volatile components such as solvents, monomers, and other low molecular weight components from the polymeric material. These volatiles are removed in one or more postreactor operations in order to recover the solvent and monomer, impart the polymer with the desired physical properties, meet environmental requirements, and eliminate residual odors and taste. In condensation polymerization, of course, the removal of volatiles is essential to drive the polymerization reaction and reach high molecular weights. Moreover, removal of volatiles, moisture and entrapped air is also essential for many types of down-stream compounding and processing equipment. 

In industrial practice, high production postreactor streams, as well as compounding and reactive processing operations, need to be devolatilized. The devolatilization process significantly affects the manufacturing cost and is critical to the quality of the product. The equipment is complex and costly and also involves the recovery of the volatiles. Todd et al. (3) and Mehta (4) reviewed, in some detail, the commercial equipment used for devolatilization, which we briefly summarize later in this Section. 

A number of alternatives can emerge, both as arrangement of internals and flowsheet configuration, such as for example, distribution of reaction and separation zones, feed policy, use of pre- or postreactors. The best alternative is selected taking into account economic and operational aspects, such as for example the minimization of byproducts and the preservation of catalyst activity over a longer period. 

Finally, the HDPE slurry from the second reactor is sent to the postreactor (3) to reduce dissolved monomer, and no monomer recycling is needed. In the decanter (4), the polymer is separated from the dispersing medium. The polymer containing the remaining hexane is dried in a fluidized bed dryer (5) and then pelletized in the extrusion section. The separated and collected dispersing medium of the fluid separation step (6) with the dissolved co-catalyst and comonomer is recycled to the polymerization reactors. A small part of the dispersing medium is distilled to maintain the composition of the diluent. 

For many systems involving gas-phase reactants, when the pressure drop is a key parameter, monolithic reactors are state of the art, whereas for the other sectors, industrial application of monoliths, in particular in postreactors, is increasing or is the subject of intense research and... 

Gas Bulk chemicals Phthalic Selective oxidation Postreactor > 2 plants 2000 Higher yields (143)... 

Postreactor, replacement Pt-gauze Instead of slurry reactor... 

The topic monolithic postreactors has dravm much attention in industry. Table 5 shows examples of commercial applications and several applications that have been patented but as far as we know not yet implemented in practice. An example of the latter is the selective oxidation of methanol resulting in high yields at lower pressure drops. 

The IMER approach does not require that the enzyme be placed in close proximity to the detector if the transducer signal is generated by a soluble product or cosubstrate of the enzymatic reaction. In the latter case, a variety of flow systems and postreactor detectors can be utilized to produce simultaneous determinations of the concentrations of several analytes. For example, an IMER can be combined with a high-performance liquid chromatography (HPLC) instrument (perhaps also in combination with mass spectroscopy) for purposes of both qualitative and quantitative analysis. The chemo-, stereo-, and regio-selectivities of enzymes facilitate separation and/or identification of analytes that may be present as different isomers (e.g., in peptide analysis based on use of peptidase IMERs in combination with these techniques to obtain structural information about the sequence of amino acids in peptides). 

There is an interior optimum. In this numerical example it occurs when 40% of the reactor volume is in the initial CSTR and 60% is in the downstream PFR. The model reaction is chemically unrealistic but illustrates behavior that can arise with real reactions. An excellent process for the bulk polymerization of styrene consists of a CSTR followed by a tubular postreactor. 

Two important types of elastomeric polyolefin blends are reactor-made iPP/ EPR blends and postreactor blend iPP/EPDM. The latter is called thermoplastic vulcanizates (TPVs), produced by dynamic vulcanization of blends containing a thermoplastic and an elastomer. To make iPP/EPDM TPV, the two polymers PP and EPDM are mixed with curatives, such as peroxides, phenolic resins, or sulfur with accelerators, and dynamically cured in an extmder resulting in a blend consisting of micrometer-sized elastomer particles dispersed in the PP matrix (20-24). Paraffinic oils are added in the melt mixing process for viscosity control and cost. In iPP/ EPDM TPV, the crystalline iPP resin is normally the minor phase. Recently, polyolefin plastomers have been added to the class of elastomeric polyolefin blends. Polyolefin plastomers are ultralow molecular weight linear low density polyethylenes (ULMW-LLDPE). Nonelastomeric polyolefin blends are blends of polyolefins with mostly nonpolyolefin (other thermoplastic) matrices as mentioned earlier. 

Gear pumps, also called melt or metering pumps, have been standard equipment for decades in textile fiber production and in postreactor polymer finishing. In the 1980s they established themselves in all kinds of extrusion lines. They consist of a pump, a drive for the pump, and pump controls, located between the screen pack (or screw) and the die. Two counter-ro-... 

Minor amounts of thermally stable neoalkoxy titanate and zirconate additives may provide a means for postreactor, in situ metallocene-like repolymerization catalysis of a filled or unfilled polymer during the plasticization phase. This may result in the creation of metallocene-like (titanocene or zirconocene) behavior associated with effects such as increased composite strain to failure resulting in increased impact toughness or enhanced polymer foamabUity. Other effects to be discussed below with specific examples are related to enhanced processability, reduced polymer chain scission, shortened polymer recrystallization time, and compatibilization of dissimilar polymers. 

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